<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
        "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<button id='btn1'>按钮一</button>
<button id='btn2'>按钮二</button>
<button onclick='demo()'>按钮三</button>


<script type="text/javascript">
    const btn1 = document.getElementById('btn1');
    btn1.addEventListener('click',()=>{
       alert("按钮1被点击了");
    });

    const btn2 = document.getElementById('btn2');
    btn2.onclick= function () {
        alert("按钮2被点击了");
    }

    function demo() {
        alert("按钮3被点击了");
    }
</script>

</body>
</html>
